The multilayer printed circuit boards have generally been manufactured by a process comprising the steps of applying an oxidizing treatment, or so-called black oxide treatment, on both sides of a core substrate board on which circuit has been formed and which is used as an interlayer circuit board (i.e., roughening the circuit surface), laying thereon one or more sheets of prepreg obtained by impregnating a thermosetting resin in a base material, further laying thereon a metal foil, and subjecting the assembly to hot pressing. The purpose of the black oxide treatment is to achieve good adhesion property between the interlayer circuit board and the prepreg. Since the interlayer circuit board without the black oxide treatment almost showed no adhesion property to the prepreg, this black oxide treatment has been an essential technique for the manufacture of multilayer printed circuit boards. This technique makes use of the oxidation phenomenon initiated by a chemical treatment of the surface copper foil of an interlayer circuit board (a copper-clad laminate in which circuit has been formed and which is used as interlayer circuit board), and practically it involves very difficult control of the process and requires a great deal of equipment investment and running cost. The fact has also been pointed out that since copper oxide has a poor acid resistance and also is low in physical strength, troubles tend to occur during the multilayer molding, drilling and through-hole plating operations.
There have also been known the multilayer printed circuit boards producing methods featuring formation of a resin layer on the surface of the interlayer circuit board, as for instance disclosed in JP-A-53-132772, JP-A-60-62194 and JP-A-63-108796. However, in any of these prior art proposals, it has been intended to improve dielectric strength, haloing resistance, or heat dispersibility, by suppressing formation of voids in the molding operation, or to increase insulating layer thickness, and those purposes are different from those of the present invention.
The present invention relates to a multilayer printed circuit board which is excellent in adhesion property of the undercoating agent to copper foil in interlayer circuit board and thereby, in which the black oxide treatment is not required. In order to solve the problems involved in the multilayer printed circuit board such as adhesion property to the copper foil in interlayer circuit board, especially that to which no black oxide treatment has been applied, heat resistance under moisture absorption, plating solution resistance, and delamination, the present inventors have made extensive researches on the composition of undercoating agent and could reach the present invention.